A vehicle includes a fuel delivery device that draws fuel from a fuel tank under pressure through a fuel pump and sends the fuel to a delivery pipe. The fuel delivery device then distributes the fuel to fuel injection valves provided in respective cylinders of an internal combustion engine. The delivery pipe of the fuel delivery device has a relief valve that opens when the pressure of the fuel (the fuel pressure) in the delivery pipe exceeds a predetermined level. This releases the fuel and lowers the fuel pressure, which is excessively high.
Particularly, an in-cylinder injection type internal combustion engine, which injects high-pressure fuel directly into cylinders, employs as the relief valve an electromagnetic relief valve that selectively opens and closes in correspondence with the energization. The electromagnetic relief valve is maintained in an open state in a certain period after the engine stops. Specifically, if the fuel pressure is maintained at a high level after stopping of the engine, the fuel may leak from a fuel injection valve and deteriorate exhaust emission caused by subsequent starting of the engine. To avoid this problem, the electromagnetic relief valve is opened after the engine is stopped, as has been described, so that the fuel pressure in a delivery pipe decreases. This reduces the amount of the fuel leaking from the fuel injection valve and prevents deterioration of the exhaust emission.
However, if the electromagnetic relief valve of the aforementioned fuel delivery device is stuck and stops functioning normally, release of the fuel through the delivery pipe cannot be performed appropriately. To solve this problem, various techniques to diagnose the operating state of electromagnetic relief valves have been proposed conventionally.
For example, a diagnosis device described in Patent Document 1 determines the difference between the temperature of the fuel in the vicinity of a delivery pipe when a fuel bypass valve, which corresponds to the aforementioned electromagnetic relief valve, is closed and the temperature of the fuel in a fuel return passage in the vicinity of the fuel bypass valve. The diagnosis device determines that the fuel bypass valve is stuck in an open state if the difference is not greater than a predetermined value. Specifically, if the fuel bypass valve is stuck in the open state, the fuel gradually flows into the fuel bypass valve after having been heated by the internal combustion engine in the vicinity of the delivery pipe. This raises the temperature of the fuel in the vicinity of the fuel bypass valve to a value approximate to the fuel temperature in the vicinity of the delivery pipe (the difference between the fuel temperature in the vicinity of the fuel bypass valve and the fuel temperature in the vicinity of the delivery pipe decreases).
However, the target of diagnosis by the diagnosis device described in Patent Document 1 is an electromagnetic relief valve that opens when the internal combustion engine is started and is maintained in a closed state when the engine operates in a normal operating state, but not the above-described electromagnetic relief valve, which is maintained in a closed state when the engine operates in a normal operating state and opens when the engine stops. It is thus desirable to provide a diagnosis device suitable for diagnosis of the electromagnetic relief valve, which is operated to open after the engine is stopped.
Patent Document 1: Japanese Patent Laid-Open Publication No. 2003-97374